CVE-2026-89637: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix UAF and buffer leak in cifschecktrans2() for malformed secondary T2

When a valid primary TRANSACT2 response has been received (mid->respbuf set, mid->multiRsp true) and a subsequent secondary response causes cifschecktrans2() to return false -- either because the SMB header is invalid (malformed != 0) or because check2ndT2() rejects the PDU -- handlemid() overwrites mid->respbuf with the new buffer (leaking the primary buffer) and, because mid->multiRsp is set, skips the server->smallbuf/bigbuf NULL-out. When the user thread frees mid->respbuf, server->smallbuf or server->bigbuf is left dangling; the demux thread reuses it for the next packet, resulting in a use-after-free.

Combine both early-exit conditions and, when mid->multiRsp is already set, abort the pending transaction inline: set multiEnd, call dequeuemid() with malformed=true, and return true so handlemid() exits without touching mid->respbuf or the server buffer pointers.